I. Hot Pressing
The hot pressing process for consolidation of ceramic powders into dense polycrystalline materials involves simultaneous application of high temperature and uniaxial pressure. It is generally applied to achieve better properties than those achievable by pressureless sintering by obtaining very low porosity, finer grain size and better homogeneity. It is especially applicable to powders which are hard to sinter to high density by conventional means (high temperature only) because of inherent refractoriness of the compounds, poor powder characteristics (e.g., large agglomerates, low surface area) or a combination of the two. Such refractory compounds may include oxides (e.g., Al.sub.2 O.sub.3, BeO), nitrides (e.g., AlN, Si.sub.3 N.sub.4, BN), carbides (e.g., SiC, B.sub.4 C, WC), and borides (e.g., TiB.sub.2) as well as multicomponent phases. Because the pressure is applied in one direction, the technique is usually limited to shapes of high symmetry (i.e., flat plates, short rods and cylinders, etc.).
FIG. 1 illustrates a simple hot pressing configuration. Graphite tooling (e.g., rams and dies) is commonly used because of the desirable physical properties possessed by graphite (i.e., high refractoriness, good machinability, good high temperature strength, low friction coefficient and low coefficient of thermal expansion) as well as its low cost and availability. Graphite tooling, however degrades in air at high temperatures and therefore requires protective non-oxidizing atmospheres (which are often also needed to prevent oxidation of non-oxide powders being processed by hot pressing).
A common problem associated with hot pressing thin pieces (e.g., less than 5 mm thick) is obtaining uniform die fill of the powder prior to processing. Non-uniform filling of the die can lead to thickness and property (e.g., density, strength) variations, as well as ram and/or die failure due to uneven stress distributions causing misalignment of the rams. Obtaining uniform die fill becomes exceedingly difficult as the area of the piece to be hot pressed increases and as attempts are made to simultaneously hot press several ceramic layers using graphite spacers between the powders (as is typically done to lower production costs). There are also problems associated with handling large quantities of powders during die filling. These include contamination from the die (particularly during the die filling step) as well as health hazards associated with airborne particulates. Another problem with hot pressing using graphite tooling is carbon contamination of the processed material. This can cause substantial discoloration of the material and can affect electrical and physical properties as well.
Powder handling problems mentioned above can be obviated to a great extent by the use of uniformly prepared, unfired sheets consisting of the powder to be hot pressed plus organic materials. These sheets, known as "green ceramic sheets", can be prepared to high uniformity by normal ceramic powder processing procedures such as extrusion or tape casting. The organic components are chosen to facilitate processing of the sheets and to impart sufficient strength to the ceramic body by binding the powder together such that it can be easily handled in its unfired form and loaded into the die. The use of green ceramic sheets is particularly beneficial if several ceramic layers are to be hot pressed simultaneously.
Besides imparting strength (and handleability) to the green ceramic sheet, a further requirement of a successful organic binder system is that it can be removed during heating such that no undesirable properties result from residues (e.g. carbon) that are left in the ceramic after densification. This attribute of a binder is commonly referred to as "clean-burning" even if the binder removal does not involve oxidation. Note that the property of clean-burning is dependent upon the environmental conditions during firing. For instance, if conventional sintering is performed in air, the binder removal is assisted by the presence of oxygen, which provides the potential for oxidation of carboneous residues from thermal decomposition. Clean-burning of the binder becomes more difficult to achieve when sintering under non-oxidizing or reducing conditions, where oxidation is effectively eliminated. Hot pressing with graphite tooling (in non-oxidizing atmospheres) exacerbates the binder removal problem, since the graphite will complete with the binder residues for any oxygen in the atmosphere. Hence, the benefits from adding small quantities of oxygen or water vapor to facilitate binder removal (as is sometimes done through the addition of water vapor) is minimized. The problem of binder removal is further exacerbated during hot pressing by the fact that the green body is enclosed by the tooling, thus requiring the binder volatiles to escape through the crack between the rams and the die. Hence, there are long diffusion paths through the green compact for the binder volatiles. There is also little chance for control of the local atmosphere in this encapsulated system. From consideration of the remarks above, it is concluded that hot pressing places more constraints on the successful binder system than conventional sintering, and thus eliminates from consideration some organics which are considered to be clean-burning when employed for conventional sintering processes.
Another common problem to be overcome in hot pressing of ceramics is sticking of the ceramic to the graphite tooling after densification. Boron nitride (BN) powder is often used as an effective release agent for hot pressing ceramics with graphite tooling. It is commonly applied to the graphite surfaces (which would contact with ceramic powder) by painting or spraying a BN-containing slurry (e.g. U.S. Pat. No. 4,518,736). BN powder is useful because it does not readily react with either graphite nor many refractory ceramics and does not readily densify itself. This, coupled with its plate-like particle morphology, allows easy delamination of the ceramic from the graphite after hot pressing.
If a thicker BN layer is necessary (because of slight reactions between the BN and other materials or because of the need for a better barrier to carbon diffusion into the ceramic), BN powder can be formed into a green sheet using the same procedures as described previously for making green ceramic sheets. The binder system used for making the BN green sheet must meet the same criteria of strength and clean-burning nature as mentioned above for the ceramic green sheets. In Japanese Published Patent Application 61-10074 a boron nitride mold releasing sheet material is used for SiC hot press sintering. The sheet material has a binder containing thermoplastic resins as its major component such as polyvinyl butyral (PVB) or polyvinyl alcohol (PVA) and a plasticizer such as butyl butylphthalylacrylate or polyethylene glycol and peptidizer such as glycerin or octadecylamine. In order for the sheet to maintain a required strength and flexibility the approximate range of the composition is given as 82-88 weight % of the mold-releasing material, 8-12 weight % of the binder, 4-6 weight % of the plasticizer, and less than 5 weight % of the peptidizing agent. For some applications, the PVB and PVA binders suggested by this Japanese patent may not be effective since they leave substantial residue upon pyrolysis in non-oxidizing atmospheres (see FIG. 2). Residues from these binders could migrate to the material to be hot pressed and affect its resultant properties.
II. Electronic Substrates
Ceramic materials are often used to support electronic components (e.g., silicon-based integrated circuits). These materials, referred to as substrates, are typically of a planar geometry and frequently have metallization on their surface(s) and/or within their interior. This metallization may serve as electrical signal pathways, ground planes, antennae, or as other passive or active electrical components in the structure. Also, the metallization may serve as a substrate for subsequent processing (e.g. brazing, plating) for adhereing the ceramic material to other materials. The ceramic materials are typically of high specific density to provide optimal thermal and mechanical performance.
There are several terms used to describe substrates and how they are made. A single layer substrate refers to a substrate with metallization patterns only on its surface(s). A multilayer substrate, in the usual sense, has internal layers of metallization patterns and can also contain surface metallization. The metallized patterns on different layers of a multilayer substrate are often interconnected by metallized through holes, or vias, in the ceramic layers. A co-fired substrate refers to one in which the green ceramic powder and metal ink pattern are bonded and densified during a single firing step. Substrates with either single or multilayer metalization are sometimes referred to in the art as single or multilayer packages, respectively.
A common approach to produce ceramic electronic substrates is by sintering ceramic (and metal) powders. This usually involves production of a sheet consisting of ceramic powder plus organic materials as described above. If an unmetallized substrate is to be produced from a green sheet, the sheet is simply heated under conditions to allow binder evolution and subsequent sintering of the ceramic powder. Unmetallized substrates are sometimes referred to as pre-fired substrates since electronic conductor patterns would be applied to the surface after the sintering of the ceramic (and usually by a separate firing step).
In a co-firing process, the metal conductor pattern is applied to the surface of the green sheet, usually as an ink or paste containing metal powder and organics, by, for example, screen printing. The metallized sheet can subsequently be fired to produce a single layer substrate with an electronic circuit pattern. In some cases, glass powder is added to the metal ink to promote mechanical bonding of the metallization to the ceramic substrate. However, this can be detrimental to the electrical conductivity of the metallization.
In a multilayer co-fired process, vias (i.e. electrical interconnections through the ceramic layers) can be formed by introducing a hole in the ceramic sheet and filling it with metal. Metallization patterns in the plane of each layer can be applied, for example, by screen printing metal inks. These metallized sheets of green tape can be laminated and fused together with sufficient pressure and minimal heat to cause the polymers in adjoining layers to bond together. The metallized laminated structure can be subsequently sintered in the manner of the single layer substrate to produce a dense multilayer substrate. A structure prepared in this manner is known as a "multilayer, co-fired" substrate.
The following discussion elaborates on some of the problems one can encounter when attempting to produce electronic substrates by sintering. One problem often encountered is uneven sintering due to inhomogeneities through the green sheet or due to uneven heating of the sheet. These problems can result in warping of the fired product, which can significantly affect yields. These problems become more pronounced as the substrate size increases.
As mentioned before, some compounds used as electronic substrates (e.g., AlN, Si.sub.3 N.sub.4) are inherently difficult to sinter. This is because of the strong covalent bonds in these materials, which leads to poor atomic mobility. Obtaining ceramics of high density from green sheets of these compounds may be impossible via pressureless sintering unless very fine powders are available (which is often not the case).
Another problem which occurs in the co-firing of ceramics with metal patterns is being unable to control the lateral shrinkage during sintering of both the metal and ceramic powders (through control of powder morphology). This can result in substantial residual stresses at the ceramic-metal interface, which can lead to warpage of the substrate and spalling of the metals. Poor control of lateral shrinkage can also result in substrates which do not meet the dimensional tolerances required. This second point can be particularly worrisome when producing many substrates, all of which must meet specified tolerances to be useful. It is also of more concern as substrate sizes increase.